This invention relates to antennas, and more particularly to a flat reflectarray antenna utilizing a polarization twist function and predetermined phase shifts to provide a directed narrow beamwidth signal.
Radar systems require some form of an antenna to produce a narrow beamwidth signal. A millimeter wave antenna has a unique requirement in that a large number of radiating elements must be integrated into a very small aperture space. Conventional corporate feed networks that are required to feed these antennas are impractical due to the extensive mechanical complexity of the network and inherent high insertion losses.
One specific type of antenna used in radar applications is the flat reflectarray antenna. This type of antenna is used for providing antenna beam collimation in place of curved, volumetric parabolic dishes because the flat surface of a reflectarray antenna can be easily stowed and deployed, and also occupies very little space. Furthermore, the flatness of such an antenna is easily maintained. However, such antennas are limited to produce a signal directed to a fixed angle.
It would therefore be desirable to provide a space fed, flat reflectarray antenna which is capable of producing a directed, narrow beamwidth signal by the selection of appropriate phase shifts.
Furthermore, it would be desirable to produce such a space-fed, flat reflectarray antenna incorporating a polarization twisting scheme to allow the reflectarray to be incorporated into a dual reflection type antenna system.
The above and other objects are provided by a space-fed, flat reflectarray antenna in accordance with the preferred embodiments of the present invention. It is a principal advantage of the antenna of the present invention that the antenna incorporates a plurality of patch antenna units formed on a thin dielectric layer. The flat reflectarray antenna is presented in an xe2x80x9cinverse Cassegrain antennaxe2x80x9d configuration and incorporates a polarization twisting scheme.
In one preferred embodiment a feed horn illuminates a subreflector. The subreflector is polarized and reflects the signal received from the feed horn back to a reflectarray element. The reflectarray element incorporates the plurality of patch antenna units and uses the patch antenna units to rotate or xe2x80x9ctwistxe2x80x9d the received signal to change the polarization of the received signal and to radiate therefrom a narrow beamwidth signal back towards the subreflector. In one preferred form the subreflector is polarized such that it reflects a vertically polarized signal but is transparent to a horizontally polarized signal, and the patch antenna units rotate the received signal from a vertically polarized signal to a horizontally polarized signal. Each of the patch antenna units includes a vertical polarization sensitive antenna and a horizontal polarization sensitive antenna. The two patch antennas are conjoined by a suitable transmission medium such as, for example, a microstrip transmission line. The length of the microstrip transmission line is selected to provide the desired degree of phase shift to the signal transmitted by the horizontal polarization sensitive patch antenna. The cumulative phase shifts thus produce a collimated antenna beam that points at a desired angle off of the boresight of the antenna.
The flat reflectarray antenna of the present invention further provides the significant benefit of being readily adapted to receive active phase shifting elements. The inclusion of active phase shifting elements enables an antenna to be constructed which is capable of electronically scanning its beam to track a desired target.